Control of electric waves



July 19, 1927. 1,636,146

F. MOHR CONTROL oF ELECTRIC wAvEs Filed July 12. 1922 Ferran/dyefes/fante by VME/N Patented July 19, 1927.

UNITED -VSTATES PATENT OFFICE.

FRANKLIN MOHR, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRICCOMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

CONTROL OF ELECTRIO WAVES.

alipncauoa ined my 12,

The present invention relates to electric wave transmission and control.More specifically it involves provisions for readily and accuratelybalancing or adjusting wave modifying circuits such as amplifiers,modulators, detectors and the like, especially of the balanced circuittype.

Balanced vacuum discharge tube circuitsi are used'for a variety ofpurposes such as amplifying waves of different frequencies withoutcombining them, modulating one wave by another while confining one ofthe Waves to the tube circuit itself, and'in other applications.Here-tofore it has been customary in setting up such circuits to select`tubes as nearly identical in their operating characteristics aspracticable and to operate the circuit with the degree .of balanceafforded b` these similar tubes. For many vpurposes tliis procedure hasproved sufficient, but there are times when a more accurate balance isdesired than can be had by merely carefully selecting the tubes.. Alsotubes frequently fail in service and need to be replaced. Circumstancesdo not always allow time for carefully matching tubes and moreover,there may not be available a number of tubes from which to make acareful selection.

It is an object of the invention tol provide for balancing'circuits ofthis general character regardless of whether the tubes themselves arelidentical or not.` It is a further object of the invention to `makereadil adjustable provisions for balancing the circuit so that 1n caseof failure of a tube a replacement can be made and the circuit can beadjusted to a balanced condition in a very small amount of time.

Specifically the means involved in carrying out the invention compriseresistances, preferably ohmic, inserted in series in the anode circuitof the discharge tubes. Experiment has shown, as will be described morefully hereinafter, that for practical purposes resistances so insertedact within considerable and ractical limits like so much resistance aded to the out ut internal impedance path of the tube itself).

in which Fig. 1 is a schematic representation of a balanced amplifiercircuit; Fi 2 is a similar representation of a balance modulatorcircuit; and Fig. 3 shows curves to be 1922. Serial No. 574,374.

explained later in connection with the operation of the circuits. 4

Referring first to Fig. 1 the balanced amplifier comprising the electrondischarge tubes 1 and 2 and their associated circuits is insertedbetween the line sections L1 and L2. These line sections may be used forthe transmission of telephone currents or other currents and it will beassumed that currents comprising a considerable range of frequencies arebeing transmitted from line section L1 to line section Lz.- Theamplifier A' is associated with the line L1 by means of the transformer.3 and with the line L2 by the transformer 4.

The operation of an amplifier of this type is Well understood by thosefamiliar with the art. The Voice currents or other -currentsto beamplified are transmitted through the coil 3 from the line section L1Land produce correspondin potential variations in the grid or inputcircuits of the tubes l and 2. Corresponding current variations ofincreased amplitude are transmitted through the condensers 8 and 9 tothe transformer 4 by which they are impressed on the outgoing line L2.It is a property of an amplifier of the balanced circuit type that ifthe circuit is accurately balanced and if a proper steady Ypolarizingpotential is maintained on the grids, waves of a number of frequenciesmay be simultaneously repeated without the production of combinationwaves due to modulating action of the amplifier tubes. Space current forthe tubes -1 and 2. is, in the case of Fig. 1, supplied througli'thechoke coils 6 and .7 to the respective anodes.

As stated above, it is important, if the waves of different frequenciesare to be repeated without the production of distorting currents due tocombinations of the repeated waves, that an accurate balance bemaintained between the tubes 1 and 2. Heretofore such balance has beenobtained by carefully selecting a pair of tubes having as nearly aspossible identical impedance and amplification characteristics. It hasbeen discovered, however, that in case the two tubes do not haveidentical impedance or Reference will now be had to the drawings lseriesin the anode circuit. Such resistances are shown at 2O and 21 and one orpreferably both of the resistances is variable to permit of adjustment.

Any one of a number of testing arrangements may be used to determinewhen the tube circuits are balanced to a high degree of accuracy. Forpurposes of illustration sources S1 and S2 of waves of dierentfrequencies are shown arranged to be connected with the input of theamplifier and a measuring circuit 17, 18 is shown for determining theamount of intermodulation occurring between the waves from these twosources for different values of the resistances 20 and 21. The wavesfrom sources S1 and S2 i should, of course, be applied to theamplifierindependently of each other and in a Way to insure that nointermodulation occurs be-` ier circuit such as would cause distortion.

in the speech transmission, a certain amount of combina-tion current of,for example, 200 cycles will be produced and this can be measured in thecircuit 17, 18.

yIn making a test to determine the degree of balance in the amplifiercircuit the switches 10 to 15 are thrown to the alter nate positionsfrom those indicated in the drawing. This resultsl in the cutting off ofthe lines L1 and L2 and in the'substitu-` tion of resistance 16 in placeof the line L'1 and the measuring circuit 17, 18 in place of the lineL2. Assuming that the sources S2 and S2 have frequencies of 800 and 1000cycles respectively, if the amplifier produces intermodulation of thesetWo waves a component of 200 cycle frequency will appear in the outputcircuit of the amplifier and will be impressed through the transformer 4and upon the measuring circuit. The circuit 17 contains a tuning orfiltering element which may be made selective to the 200 cycle frequencywave but which will be arranged not to transmit the current of 800cycles or 1000 cycles or other frequencies. The circuit 17 is alsoassumed to contain a suitable integrating device, as for example, anordinary vacuum tube detector to render the 200 cycle wave capable ofaffecting the meter 18. With the circuits so arranged the resistances 20and -21 are adjusted by trial so that preferably the amount of externalresistance included in one of the anode circuits is a minimum, and sothat just suiicient resistance is included in the other anode circuit togive the minimum reading -of the meter 18.

In Fig. 2 the balanced modulator is shown arranged `for modulating acarrier wave from a source C S in accordance with currents received fromthe line L3 and for impressing the modulated wave on the outgoing vlinesection L4. With the carrier wave source C S connected as shown incommon to both grids of the modulator tubes it is a property of thiscircuit, when accurately balanced, to suppress transmission of theunmodulated carrier wave to the line L4 but to permit transmission ofthe modulated wave components. Any unbalance in the modulator circuitresults in the transmission to the line L4 of some of the unmodulatedwaves from the source C S. The output side of the tube circuit of Fig. 2has a different arrangement from that of Fig. 1, although either type ofanode circuit is suitable for either the amplifier or the modulator. InFig. 2 the plate current is supplied from the battery indicated, througha single choke impedance 22 to the midpoint of the primary winding ofthe output transformer.

In order to adjust the circuit of Fig. 2 for an accurate balance theswitches 10, 11, 14 and 15 are thrown to the alternate positions fromthose in which they are shown in the drawing. This substitutesresistance 16 for line Ls and the measuring circuit for the line L4. Themeasuring circuit comprises the thermo-couple TC and a meter 18 whichmay be any suitable type of current meter. vWith the connections so madeand the/source CSvoperating, the resistances 20 and 21 areI adjusted togive a minimum reading on meter 18 and as in the above described casethe resistances are preferably so adjusted that one of them ispractically all cut out of the anode circuit.

Fig. 3 shows experimental curves obtained with a typical balanced tubecircuit. In this figure the abscissae represent the ratio of resistance20 or 21 that is being used r to the nominal anode-cathode impedance ofone of the tubes, expressed in per cent, while the ordinates representthe 'percentage of efficiency or distortion of the circuit as the casemay be, based on the etiiciency or distortion of the circuit whenresistances 20, 21 are` at zero value. With both resistances 20 and 21at zero the circuit is in the unbalanced condition and the distortion'is for that case, assuming that the tubes are not identical. As eitherresistance is ins erted in the anode circuit to reduce the amount ofdistortion the eiiciency of the circuit as a whole is also reduced, butfrom the relation of the curves, it will be seen that only a slightsacrifice in efficiency is necessary in order to reduce the distortionvery greatly. i

The invention is not to be construed as limited to the particular formsof embodiment that have been shown and described, since it is capable ofuse generally. Its scope is defined in the claims.

What is claimed is: k l

l. The method of purifying the output wave of a wave combining circuitemploying a pair of balanced space discharge tubes, which comprisescontrolling the amount of ohmic resistance in the output path of saidwave 4in relation to the internal impedance of said tubes to neutralizein said output path any wave components present due to slight unbalanceof said tubes.

2. The method of combining waves of a plurality of frequencies in amodulator circuit employing a pair of space discharge tubes connected inbalanced circuit relation which comprises varying the ohmic resistancein series in the circuits external to said tubes in relation to theeectiveA internal impedances of said tubes -to suppress in the outgoingvcircuit undesired components of said Waves due to slight differences insaid eii'ective internal impedances of said tubes.

3. In a wave modifying circuit employing a pair of space discharge tubesconnected in balanced circuit relation, means for supplying waves ofdifferent frequencies to said circuit, anode-cathode branches tor eachof said tubes, an outgoing circuit connected to said anode-cathodebranches, and means for controlling the ohmic resistance in saidcathode-anode branches to suppress in said outgoing circuit componentsof said Waves due to slight differences in the impedance characteristicsof said tubes.

4:. A wave modifying circuit comprising a pair of space discharg'etubes,each having cathode, anode and grid elements, con-- trol circuits forsaid grids, a cathode-anode branch for each tube, an outgoing circuitconnected to said cathode-anode branches, means for supplying waves ofdifferent frequencies to the grid circuits of said tubes, and regulatingmeans to Suppress in said outgoing circuit components of said waves dueto slight differences in the eiective impedances of said tubescomprising an ohmic resistance in the anode circuit of one of saidtubes.

5. A'wave modifying circuit comprising incoming and outgoing circuits,means for supplying waves or' a plurality of frequencies to the incomingcircuit, a differential circuit inserted between said incoming andoutgoing circuits comprising a, pair ofelectron discharge devices, eachhaving cathode, anode and grid elements, said cathode and grid elementsbeing associated with said incoming circuit, anode branches including aconnection from each anode to the outgoing circuit and to said cathodes,and variable ohmic resistance means in said outgoing-circuit i'orvarying the total impedance of one anode branch with respect to the.other to neutralize in said outgoing circuit undesired wave componentsdue to dierences in the eti'ective internal impedances of said tubes.

6. A balanced modulator circuit comprising `a pair of electron dischargetubes, each having a cathode, a grid and an anode, means forsimultaneously supplying waves of different frequencies to the grid andcathode elements of said tubes, an outgoing cirsis'tance means in saidoutgoing circuit for suppressing in the outgoing circ-uit unde-` sired-unmodulated components of said waves resulting from lack of completebalance of said tubes.

7. A balanced modulator circuit comprising a pair of electron-dischargetubes having cathode, anode and grid elements, means for simultaneouslysupplying waves of different frequencies to said grid and cathodeelements of said tubes, one of said waves being impressed on said gridelements in the same sense, anode-cathode branches, an outgoing circuitconnected to said anode-cathode branches, and means for suppressing insaid outgoing circuit unmodulatedl components of the wave impressed onsaid grid elements in the same sense due to unbalance between said tubescomprising Variable ohmic resistance in series in said anode-cathodebranches. n

In witness whereof, I hereunto subscribe my name this 10th day of JulyA. D.,1922.

- FRANKLIN MOHR.

'cuit for saidA tubes, and variable ohmic re-

